DESCRIPTION (Verbatim from the Applicant's Abstract): Positron Emission Tomography (PET) is an in vivo analog of autoradiography, which makes it a potentially powerful new tool for imaging biological processes in small laboratory animals. Its critical advantage is that functional data can be obtained non-invasively, allowing each animal to be studied repeatedly. Thus, each animal can serve as its own control in studies with a longitudinal design. Recently, the demand for small animal PET is also driven by the pharmaceutical industry where in vivo quantification of biological processes to measure an agent's mechanism of action and its concentration at the site of action is necessary. While the advantages of small animal imaging with PET are obvious, the challenges are also very significant. The main barriers to using PET in studies of laboratory animals have traditionally been poor spatial resolution, low sensitivity, and high cost. To address these deficiencies, we propose to develop a detector that can achieve extremely fine spatial resolution and high sensitivity in a cost-effective manner. The detector will be based on finely pixellated monolithic block of scintillator coupled to a position sensitive detector. Phase II efforts will focus on the development of a complete small-animal PET system as a prototype product. PROPOSED COMMERCIAL APPLICATION: The demand for small animal PET imagers is being driven both by the biomedical research community and by the needs of the pharmaceutical industry. In addition to the small animal PET systems, the detector technology proposed here have direct applications in other areas of nuclear imaging including whole body PET scanners, gamma cameras, intraoperative probes, and breast imaging. Additional applications include nuclear physics, and astronomy. Together these applications represent a large dollar market, a significant fraction of which would benefit from this research.